Attempts to revive controls on guns like the AR-15 have failed in the face of objections from the powerful gun lobby. Photograph: Rick Gershon/Getty Images

James Holmes is responsible for one of the largest mass shootings of its kind in US history – but it did not result in the largest number of deaths.

The numbers may yet change, but of the 71 people shot at a Colorado cinema, 12 were killed. That is perhaps because one of Holmes's weapons was a shotgun that could wound rather than kill at longer distances. Doctors were picking pellets, not bullets, out of some of the victims.

Those who died may well have fallen victim to a powerful assault rifle – modelled on the US army's M-16 that was once restricted – that Holmes was also carrying, as well as a Glock handgun, the best-selling pistol in the US after it was made popular in films. The assault rifle was fitted with a drum magazine capable of holding more than 100 bullets.

The police said he appeared to have legally purchased all of the weapons, including a second pistol found in Holmes's car.

The guns were bought over the last two months at several shops including a camping and hunting chain, Bass Pro Shops, and Gander Mountain Guns. Both outlets released statements on Friday.

Bass Pro in Denver, where Holmes purchased the shotgun on 28 May and one of the Glocks on 6 July, said: "Based on the records we have reviewed, personnel in our Denver store correctly and fully followed all federal requirements with respect to the sale of one shotgun and one handgun to the individual identified in this incident. Background checks, as required by federal law, were properly conducted and he was approved."

It added that the staff's "hearts go out to the victims and their families".

Holmes bought the other Glock on 22 May from Gander Mountain's Aurora store and the AR-15 on 7 June from its Thornton outlet, law enforcement officials told the AP. But the company refused to say if they had sold Holmes any guns, despite acknowledging that the store did have that information.

A statement read: "Gander Mountain fully cooperates with law enforcement in criminal investigations like the one regarding the tragic events in Colorado. We operate in strict compliance with all local, state and federal laws regarding firearms ownership and are fully cooperating with this on-going investigation."

It added: "We send our thoughts and prayers out to those affected. In respect for the victims of this tragic crime and their families, we will have no further comment in this matter."

Colorado state law does not require a licence to own the firearms, but Holmes would have undergone a federal background check. He would have passed because the police said he had no more than a driving offence on his record.

Shotguns and handguns have always been readily available in Colorado, but until eight years ago Holmes would not have been able to buy the AR-15 semi-automatic assault rifle he was carrying quite so easily.

The manufacture and import of AR-15s and similar weapons, such as AK-47s, were banned in the US in 1994. There were also limits on the size of magazines that could be fitted, limiting them to holding no more than 10 bullets.

Those prohibitions fell away 10 years later, and attempts to revive them have failed in the face of objections from the powerful National Rifle Association allowing Holmes not only to purchase the powerful weapon but also to fit it with the magazine drum holding a large number of bullets.

The Brady Campaign to Prevent Gun Violence gives Colorado a score of just 15 points out of 100 in assessing adequate gun control laws. It criticises the state for not restricting bulk purchases of weapons, for failing to record identification of weapons so they can be traced, for failing to require any permits to buy a weapon and for permitting large magazines of the kind used by Holmes.

Colorado permits people to carry loaded and concealed weapons with a licence. The NRA is fighting a legal case against the requirement for a permit.

The Brady Campaign says the state has "weak gun laws that help feed the illegal gun market and allows the sale of guns without background checks and put children at risk".